Fast-forward existing behind-only branches during PR checkout

[Copilot]

Checkout Pull Request created a pr/<author>/<number> branch whenever the local and remote SHAs differed—even when the existing local branch was only behind. This left the expected branch stale and associated the PR with the generated branch.

• Checkout behavior
  • Reuse the local branch when it is behind-only and tracks the PR’s remote head.
  • Check out and pull that branch instead of creating a generated PR branch.
  • Preserve the existing unique-branch behavior for divergent or unrelated branches.

const canFastForward = branch.behind > 0
  && branch.ahead === 0
  && branch.upstream?.remote === remoteName
  && branch.upstream?.name === originalBranchName;

• Regression coverage
  • Verify a behind-only branch is checked out, pulled, and associated with the PR without creating pr/<author>/<number>.

• Fixes "Checkout Pull Request" creates a pr/<author>/<number> branch when the local branch is only behind the remote #8900

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR updates the “Checkout Pull Request” flow to reuse an existing local PR head branch when it’s only behind the remote (safe fast-forward), instead of always creating a generated pr/<author>/<number> branch whenever SHAs differ. It also adds regression tests to ensure behind-only branches are pulled and associated with the PR without generating a new branch.

Changes:

• Reuse and fast-forward an existing behind-only local branch that tracks the PR head, rather than creating pr/<author>/<number>.
• Add tests covering: behind-only branch checkout + pull, behind-only already-checked-out pull, and a worktree scenario where a unique branch is used.

Show a summary per file

File	Description
src/github/pullRequestGitHelper.ts	Adds “behind-only + tracking” detection to allow fast-forwarding the existing branch during PR checkout.
src/test/github/pullRequestGitHelper.test.ts	Adds regression tests validating the new checkout/fast-forward behavior and worktree handling.

Review details

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite